Many devices fabricated as micro-machined electromechanical systems (MEMS), both sensor and actuator devices, and methods for manufacturing the same are generally well-known. See, for example, U.S. Pat. No. 6,642,067, METHOD OF TRIMMING MICRO-MACHINED ELECTROMECHANICAL SENSORS (MEMS) DEVICES, the complete disclosure of which is incorporated herein by reference, that describes a MEMS acceleration sensor and method for manufacturing the same. In another example, U.S. Pat. No. 6,428,713, MEMS SENSOR STRUCTURE AND MICROFABRICATION PROCESS THEREFORE, issued to Christenson, et al. on Aug. 6, 2002, which is incorporated herein by reference, describes a capacitive acceleration sensor formed in a semiconductor layer as a MEMS device. Other known MEMS devices include, for example, micro-mechanical filters, pressure sensors, gyroscopes, resonators, actuators, and rate sensors, as described in U.S. Pat. No. 6,428,713.
Vibrating beam acceleration sensors formed in a silicon substrate as MEMS devices are also generally well-known and are more fully described in each of U.S. Pat. No. 5,334,901, entitled VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,456,110, entitled DUAL PENDULUM VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,456,111, entitled CAPACITIVE DRIVE VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,948,981, entitled VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,996,411, entitled VIBRATING BEAM ACCELEROMETER AND METHOD FOR MANUFACTURING THE SAME; and U.S. Pat. No. 6,119,520, entitled METHOD FOR MANUFACTURING A VIBRATING BEAM ACCELEROMETER, the complete disclosures of which are incorporated herein by reference. Such vibrating beam accelerometers have been fabricated from a body of semiconductor material, such as silicon, using MEMS techniques. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, entitled METHOD OF MAKING AN ELECTROSTATIC SILICON ACCELEROMETER, and U.S. Pat. No. 4,945,765, entitled SILICON MICROMACHINED ACCELEROMETER, the complete disclosures of which are incorporated herein by reference.
As is generally well-known, a typical MEMS device, whether a sensor or an actuator, has a size on the order of less than 10−3 meter, and may have feature sizes of 10−6 to 10−3 meter. Moving parts within a device are typically separated by microscopically narrow critical gap spacings, and as such are highly sensitive to particle contamination, such as dust and other microscopic debris. MEMS devices are also sensitive to contamination arising from corrosive environments; humidity and H2O in either the liquid or vapor phase, which may cause stiction problems in the finished device; and mechanical damage such as abrasion. MEMS devices are often required to operate at a particular pressure or in a vacuum; or in a particular liquid or gas such as, for example, dry nitrogen; and in different acceleration environments from high-impact gun barrel munitions to zero gravity deep space applications. Such application environments aggravate the device sensitivity to contamination.
The manufacture of MEMS devices includes many individual processes. Each of the individual processes may expose the device to a source of contamination. This sensitivity to particle contamination poses a challenge to the structural design and microfabrication processes associated with these small-scale, intricate and precise devices in view of the desire to have fabrication repeatability, fast throughput times, and high product yields from high-volume manufacturing. MEMS devices are typically encapsulated and hermetically sealed within a microshell, i.e., between cover plates. The microshell serves many purposes, including shielding the micro-mechanical parts of the MEMS device from damage and contamination.
Traditionally, MEMS devices utilize a wafer stack or “sandwich” design of two or three stacked semiconductor silicon wafers, with the sensor or actuator device mechanism wafer being positioned in the center between two outside cover wafers or “plates” in a three-wafer device. In a two-wafer device, a single cover plate is mounted on top of the mechanism wafer. The cover plates are bonded to the mechanism wafer in a three dimensional MEMS device. A frit glass seal or another mechanism bonds the cover plates to the mechanism wafer along their common outer edges or peripheries and hermetically seals the device. Other common bonding mechanisms include, for example, eutectic metal-to-metal bonding, silicon-to-silicon fusion bonding, electrostatic silicon-to-silicon dioxide bonding, and anodic bonding for silicon-to-glass bonds. These conventional bonding mechanisms also result in a hermetically sealed device. The cover plate wafer or wafers act as mechanical stops for movable portions of the mechanism wafer, thereby protecting the mechanism wafer from forces that would otherwise exceed the device's mechanical limits.
Electrical connections to the sensitive portions of the mechanism wafer require one or more bond wires that pass through window apertures in one cover plate and connect to conductive paths formed on the surface of the mechanism wafer. These conductive paths and the corresponding windows in the cover plate have traditionally been located within the interiors of the respective mechanism and cover wafers, thus being interior of the seals that bond the cover plates to the mechanism wafer along their respective peripheral edges. These internal windows can allow particulate contamination or moisture to invade the interior of the MEMS device during handling, transportation, testing or wire bonding operations, which can result in premature failure.